There have been provided particle counters as devices for measuring particles in fluids, which are, for example, liquids such as a chemical solution and water, or gas such as air. At the particle counter, a fluid containing particles is irradiated with a laser beam. Scattered light from the particles in the fluid is observed to count the particles (for example, refer to Patent Literature 1).
For example, in the manufacture of semiconductor wafers, the particles of impure substances contained in the chemical solution to be used exert influence on the manufacturing process. Accordingly, a particle counter is used to count the particles in the chemical solution to control the state of the chemical solution. In the chemical solution, scattered light (background light) and the like are generated by the medium (namely, the chemical solution itself). Thus, measuring particles in the chemical solution involves larger background noise than that in the case of measuring the particles in the water. This makes it difficult to count small-size particles (for example, 30 nm or less).
One particle counter uses a multi-divided light receiving element. The multi-divided light receiving element reduces the effective light receiving areas at end portions. As a result, reducing the noise resulting from the background light improves the signal-to-noise (S/N) ratio (for example, refer to Patent Literature 1).
Meanwhile, there has been proposed a dynamic light-scattering measuring device having a Mach-Zehnder interferometer and a low-coherence light source (for example, refer to Patent Literature 2). Such a dynamic light-scattering measuring device obtains a particle size distribution based on changes in scattered light intensity resulting from the Brownian motion of the particles.